1. Field of the Invention
The invention relates to temperature-stabilized reference voltage circuits, and more particularly to a sub-1-V bandgap reference circuit using a low supply voltage.
2. Description of the Related Art
Reference circuits are necessary in many applications ranging from memory, analog, mixed-mode to digital circuits. The demand for a low voltage reference is especially apparent in mobile battery-operated products. Low voltage operation is also a trend of process technology advancement. It is difficult to approach a stable operation in conventional bandgap reference (BGR) circuits when the supply voltage is under 1.5 V. As a result, the demand for a new bandgap reference circuit technique which is stable and operated at low supply voltages is inevitable.
For a discussion of bandgap reference circuits with below 1.5 V power supply voltages refer to:
                H. Banba, H. Shiga, A. Umezawa, T. Miyaba, T. Tanzawa, S. Atsumi, and K. Sakui, “A CMOS Bandgap Reference Circuit with Sub-1-V Operation,” in IEEE Journal of Solid-State Circuits, Vol. 34, No. 5, pp. 670–673, May 1999, which describes a BGR circuit where Vref has been converted from the sum of two currents; one is proportional to Vf and the other is proportional to VT, and        J. Doyle, Y. J. Lee, Y.-B. Kim, H. Wilsch, and F. Lombardi, “A CMOS Subbandgap Reference Circuit With 1-V Power Supply Voltage,” in IEEE Journal of Solid-State Circuits, Vol. 39, No. 1, pp. 252–255, January 2004, where threshold voltage reduction and subthreshold operation techniques are used. Large ΔVBE (100 mV) as well as a 90-dB operational amplifier are used to circumvent the amplifier offset.        
Shown in FIG. 1a is one example of a conventional CMOS BGR circuit which is composed of a CMOS op-amp OA1, a current mirror comprising MP1, MP2, MP3, diode-wired transistors Q1, Q2, Q3, and resistors R1, R2, all implemented in the standard CMOS process. VDD and VSS are the power supply rails. The area ratio of Q1, Q2, Q3 is Q1:Q2:Q3=1:M:1. Transistors MP1, MP2, MP3 supply currents I1, I2, I3, respectively. The voltage VBE1 is seen at node BE1, voltage VN1 is seen at node N1, voltage VBE2 is seen at node VBE2, and voltage VBGR is seen at output node BGR.
The current versus voltage relation of a general diode is expressed as:
                              I          D                =                              I            S                    ·                      (                                          ⅇ                                                      q                    ·                                          V                      D                                                                            k                    ·                    T                                                              -              1                        )                                              (        1        )            If
            V      D        ⪢          kT      q        ,then eq. (1) can be approximated as
                              I          D                ≅                              I            S                    ·                      ⅇ                                          q                ·                                  V                  D                                                            k                ·                T                                                                        (        2        )            solving for VD:
                              V          D                =                                                                              k                  ·                  T                                q                            ·              ln                        ⁢                                                  ⁢                                          I                D                                            I                S                                              =                                                    V                T                            ·              ln                        ⁢                                                  ⁢                                          I                D                                            I                S                                                                        (        3        )            where    k is Boltzmann's constant (1.38×10−23 J/K),    q is the electron charge (1.6×10−19 C),    T is the absolute temperature (K),    VD is the voltage across the diode,    ID is the diode current,    IS is the saturation current, and    VT is the thermal voltage=(k·T)/q.The PMOS transistor dimensions of MP1, MP2, and MP3 are the same. Therefore the currents I1, I2, and I3 have the same value because their gates are connected to a common node.
                                          (                          W              L                        )                    MP1                =                                            (                              W                L                            )                        MP2                    =                                    (                              W                L                            )                        MP3                                              (        4        )                                          I          1                =                              I            2                    =                                    I              3                        =            I                                              (        5        )            using (3) and (4), VBE1 and VN1 in FIG. 1a can be expressed as:
                              V          BE1                =                              V            T                    ⁢                                          ⁢          ln          ⁢                                          ⁢                      I                          I              S                                                          (        6        )                                          V          N1                =                              I            ·            R1                    +                                                    V                T                            ·              ln                        ⁢                                                  ⁢                          I                              M                ·                                  I                  S                                                                                        (        7        )            where M is the area ratio between diodes Q1 and Q2 (Q1:Q2=1:M; thus M=Q2/Q1) and where VBE1 is the base-emitter voltage drop of a bipolar transistor or the diode turn-on voltage. Because VBE1 and VN1 are a pair of input voltages for the op-amp, they are controlled to be the same voltage.VBE1=VN1  (8)Using (6), (7), and (8), I is given by:
                    I        =                                            V              T                        R1                    ·          ln          ·          M                                    (        9        )            Using (9), the conventional BGR, the output voltage VBGR becomes
                              V          BGR                =                                            I              ·              R2                        +                          V              BE1                                =                                                    R2                R1                            ·                              V                T                            ·              ln              ·              M                        +                          V              BE1                                                          (                  10          ⁢          a                )            Where VBE1 has a negative temperature coefficient of about −1.5 mV/K as shown in FIG. 1b, whereas VT has a positive temperature coefficient of about +0.087 mV/K, so that VBGR is determined by the resistance ratio of R2/R1 and the area ratio of diode-wired transistors Q1, Q2. FIG. 1b is a graph of the simulation results of the prior art bandgap circuit relating temperature in ° C. on the horizontal axis to voltage in Volt on the vertical axis for Curve VBE1 and Curve VBGR (output voltage). Thus VBGR is controlled to be about 1.25 V where the temperature dependence of VBGR becomes negligibly small. As a result, the supplied voltage can not be lower than 1.25 VDS+VDS3, which limits the low voltage design for CMOS circuits as shown in FIG. 1c, Curve 1. FIG. 1c is a graph of the simulation results of the prior art bandgap circuit relating the supply voltage VDD in Volt on the horizontal axis versus the bandgap reference output voltage VBGR in Volt on the vertical axis.
A review of the prior art U.S. patents has yielded the following related patents:    U.S. Pat. No. 6,788,041 (Gheorghe et al.) discloses a bandgap reference circuit which when operating with a voltage source in the range from 1.0 to 1.2 volt provides a Vref output of about 242 and 245 mV, respectively, utilizing a PTAT current source.    U.S. Pat. No. 6,605,987 (Eberlein) teaches a temperature-stabilized reference voltage circuit using the current-mode technique, in which two partial currents are superimposed on each other and converted into the reference voltage. The circuit permits the implementation of low temperature-compensated output voltages below 1.0 V.    U.S. Pat. No. 6,529,066 (Guenot et al.) shows a bandgap circuit producing an output of 1.25 V and utilizing parasitic vertical PNP transistors operating at different current densities. A difference in the base-emitter voltages is developed across a resistor to produce a current with a positive temperature coefficient. When combined with another voltage with a negative temperature coefficient a bandgap reference voltage is produced.    U.S. Pat. No. 6,566,850 (Heinrich) describes a bandgap reference circuit, which includes a sensing circuit and a current injector circuit, that can transition quickly to a desired operational state by injecting bootstrap current into an internal node of the bandgap reference circuit. The bandgap reference circuit is effective with a low voltage power supply (e.g., 1–1.5 V).    U.S. Pat. No. 6,531,857 (Ju) presents a bandgap reference circuit which has a segmented resistor coupled across the emitter-base terminals of a PNP transistor to generate a VBE current. The resistor sums this VBE current with a PTAT current and generates a Vref voltage, where Vref can be less than VEB. VEB typically is less than or equal to 0.7 V, resulting in a VDD voltage of equal or larger than 0.85 V.    U.S. Pat. No. 6,489,835 (Yu et al.) discloses a bandgap reference circuit which operates with a voltage supply that can be less than 1 V and where only one non-zero current operating point is available. The bandgap reference circuit comprises a core circuit with an embedded current generator, and a bandgap reference generator with output VBG.    U.S. Pat. No. 6,281,743 (Doyle) describes a sub-bandgap reference circuit yielding a reference voltage smaller than the bandgap voltage of silicon. The generation of the reference signal includes generating first and second signals with negative and positive temperature coefficients, respectively. The first and second signals are then sampled and stored on first and second capacitors. A low impedance path between these capacitors yields the reference signal. Simulation shows a stable sub-bandgap reference output of 0.605 V using a supply voltage of only 1 V.    U.S. Patent Application Publication US 2004/0169549 A1 (Liu) presents a bandgap reference circuit comprising an op-amp, a plurality of MOS transistors coupled to the op-amp, a plurality of resistors and bipolar transistors coupled to the MOS transistors. Simulation and measurement results indicate that Vref, generated by the bandgap reference circuit, is within the range of 1.18 to 1.2 V from −40° C. to 120° C.    U.S. Patent Application Publication 2004/0155700 A1 (Gower et al.) teaches a bandgap reference voltage generator with low voltage operation comprising a first closed-loop circuit having a first current with a positive temperature coefficient, and a second closed-loop circuit having a second current with a negative temperature coefficient. The bandgap reference voltage generator includes a multitude of output stages where each output may be independently scaled to have either a zero, a positive or a negative temperature coefficient.
A problem of many of the prior art circuits is that they tend not to be stable until the supply voltage is larger than 1.5 V or require additional components, such as capacitors which take considerable area, for stable operation at low supply voltages. Clearly a BGR circuit is desirable which can work down to sub-1-V supply voltages which is stable, simple to integrate, and has low cost.